The present invention, in some embodiments thereof, relates to data communication networks and, more particularly, but not exclusively, to systems and methods for cancellation of interference signals transmitted by interference transmitters.
Wireless communication network (e.g., cellular networks) receivers are susceptible to interference from different sources. One type of interference originates from a foreign interferer, a transmitter that is not part of the immediate network. The interference may originate, for example, from a remotely located transmitter, carried by atmospheric propagation such as a tropospheric duct, or from a near-by located transmitter that transmits on frequencies that are overlapping and/or adjacent to the spectrum used by the interfered receiver. Remote interferers may broadcast transmissions such as analog or digital TV from other countries.
The transmission may occur on the same channel as the cellular network, for a variety of reasons for example, due to lack of harmonization between countries with respect to use of spectrum, or through remote time-division duplex (TDD) base stations that exhibit large propagation delays which hamper the uplink/downlink duplexing separation that was planned through synchronized TDD.
There are several scenarios that may result in unwanted interference at the received spectrum of a base-station or access point. One example is intentional interference, also known as jamming. Such interference is created intentionally in order to harm a wireless network and prevent communication. Jammers may be employed in defense scenarios and the proposed solution may be used to protect a communication system against hostile jammers.
In civilian communication the usage of jammers is rare and in many cases the interference is caused unintentionally. For example, the interference may be a result of transmission in a different spectrum resulting in interference due to radiofrequency (RF) impairments in the desired signal spectrum. Several reasons for interference include, for example, differences in usage of licensed spectrum among countries, usage of Time Domain Duplexing schemes, and usage of unlicensed spectrum.
In different countries, the same parts of the spectrum may be used for different applications. For example, in one country a certain spectrum is used for uplink cellular communications (communication from the users to the base-station) while in another country the same spectrum is used for TV broadcasting. This scenario may result in strong interference signals being generated by the broadcast transmitters when received by a cellular base-station. Such phenomenon may be significantly intensified by tropospheric ducts that may extend the range of the interfering signals by hundreds or even thousands of kilometers. As a result, interference due to lack of spectral harmonization may be present between countries which do not have a common border.
In another example, self interference may be created by a Time Domain Duplexing (TDD) regime network among sites separated by large distances during strong propagation created by tropospheric ducts. A tropospheric duct carrying a signal beyond the optical horizon may result in unwanted interference where the network sites are synchronized to a specific downlink/uplink framing in time so that all the network base-stations are transmitting or receiving at the same time. When weather conditions create strong duct propagation, these TDD signals may travel for several tens or hundreds of kilometers, and hit another site of the same TDD network. The time delay over the duct can be several hundreds of micro-seconds or even more than one millisecond. The delay may result in downlink signals transmitted from one site during downlink frame time, being received during uplink frame time at a site located tens or hundreds of kilometers away.
Another example of interference created in TDD network implementations, are cases where different or unsynchronized TDD networks are used within the same or neighboring areas. For instance, two operators using different downlink/uplink ratios, or even the same ratio but out of phase might interfere with each other if they have adjacent spectrum or the same spectrum across two sides of a common border.
In the case of two TDD technologies (e.g., WiMAX and TD-LTE) being used in the same geographic area, the different frame structures make it impossible to synchronize them, and hence interference is inevitable.
In yet another example of interference resulting from different spectral regimes between countries, is a case where a part of the spectrum is licensed in one country, but unlicensed in another country. For example, the spectrum between 902 Mega-Hertz (MHz) and 915 MHz is part of the GSM uplink band used in Europe, The Middle East and other parts of the world. In North America this spectrum is part of the ISM license free band and used for cordless phones, surveillance cameras and other consumer applications.
As a result, equipment purchased in North America operating in the 900 MHz ISM band that is then brought to and used in Europe or other areas where the 900 MHz spectrum is used for cellular communication, may result in unwanted interference in base-station sites.
Yet another case of interference generated by misuse of equipment in licensed frequencies, are oscillating repeaters/bi-directional amplifiers, due to lack of isolation between their service and donor antennas.
When using unlicensed spectrum, interference can be expected from other communication systems sharing the same spectrum. Methods for avoiding interference, for example, retransmission MAC, frequency hopping, and other methods, may enable reliable communication over unlicensed frequency at the expense of throughput and capacity.